Zoneless tachyarrhythmia detection with real-time rhythm monitoring

ABSTRACT

A method of using an implantable medical device (IMD) to monitor a ventricular contraction rate of a subject, monitor an atrial contraction rate of the subject, declare tachyarrhythmia if the ventricular contraction rate exceeds the atrial contraction rate, and declare a slow tachyarrhythmia when the ventricular rate exceeds the atrial rate and the ventricular rate is less than a specified maximum pacing rate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of U.S. application Ser. No. 11/301,716filed on Dec. 13, 2005, now U.S. Pat. No. 7,702,384, which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The field generally relates to implantable medical devices and, inparticular, but not by way of limitation, to systems and methods fordetecting tachyarrhythmia in a patient.

BACKGROUND

Implantable medical devices (IMDs) are devices designed to be implantedinto a patient. Some examples of these devices include cardiac functionmanagement (CFM) devices. CFMs include implantable pacemakers,implantable cardioverter defibrillators (ICDs), cardiacresynchronization therapy devices, and devices that include acombination of such capabilities. The devices are typically used totreat patients using electrical therapy and to aid a physician orcaregiver in patient diagnosis through internal monitoring of apatient's condition. The devices may include electrical leads incommunication with sense amplifiers to monitor electrical heart activitywithin a patient, and often include sensors to monitor other internalpatient parameters. Other examples of implantable medical devicesinclude implantable insulin pumps or devices implanted to administerdrugs to a patient.

Additionally, some IMDs detect events by monitoring electrical heartactivity signals. In CFM devices, these events include heart chamberexpansions or contractions. By monitoring cardiac signals indicative ofexpansions or contractions, IMDs are able to detect tachyarrhythmia.IMDs are further able to provide therapy for tachyarrhythmia, such ashigh energy shock therapy or anti-tachycardia pacing (ATP).Tachyarrhythmia includes abnormally rapid heart rate, or tachycardia,including ventricular tachycardia (VT) and supraventricular tachycardia.Tachyarrhythmia also includes rapid and irregular heart rate, orfibrillation, including ventricular fibrillation (VF). Typically, ICDsdetect tachyarrhythmia by first detecting a rapid heart rate. Otherdetection methods in addition to fast rate detection are used to reducethe incidence of inappropriate shocks. The present inventors haverecognized a need for improved sensing of events related to devicerecognition of tachyarrhythmia.

SUMMARY

This document discusses, among other things, systems and methods fordetecting events related to cardiac activity. A system example includesan implantable medical device (IMD). The IMD includes a ventricularcontraction sensing circuit that provides a sensed ventricularcontraction signal, a timer circuit that provides a ventricular timeinterval between ventricular contractions, and a controller circuitcoupled to the timer circuit, the controller circuit determines theventricular contraction rate using the ventricular time interval. Thecontroller circuit further includes a tachyarrhythmia detection modulethat declares tachyarrhythmia, in response to detecting a sudden rateincrease, without comparing a ventricular rate or time interval to arespective tachyarrhythmia detection rate or time interval threshold.

A method example includes using an IMD to monitor ventricularcontraction intervals (V-V intervals) of a subject, detect a sudden rateincrease in the V-V intervals, determine that the sudden rate increaseis sustained for a specified period of time, and deem the sudden rateincrease indicative of tachyarrhythmia.

Another method example includes monitoring a ventricular contractionrate of a subject, monitoring an atrial contraction rate of the subject,and declaring tachyarrhythmia if the ventricular contraction rateexceeds the atrial contraction rate.

This summary is intended to provide an overview of certain subjectmatter of the present patent application. It is not intended to providean exclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the subjectmatter of the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of portions of a system that uses animplantable medical device.

FIGS. 2A-B illustrate implantable medical devices coupled by one or moreleads to a heart.

FIGS. 3A-B illustrate an implantable medical device that does not useintravascular leads.

FIG. 4 shows an example of portions of a system that detectstachyarrhythmia.

FIG. 5 illustrates graphs of ventricular contraction intervals.

FIG. 6 shows another example of portions of a system that detectstachyarrhythmia.

FIG. 7 is a block diagram of an example of a method of using animplantable medical device.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and specific examples inwhich the invention may be practiced are shown by way of illustration.It is to be understood that other examples may be used and structural orlogical changes may be made without departing from the scope of thepresent invention.

This document discusses systems and methods for improved detection ofcardiac events. Because it is assumed that tachyarrhythmia isaccompanied by fast heart rates, implantable medical devices (IMDs),such as implantable cardioverter defibrillators (ICDs) for example,typically detect tachyarrhythmia when a heart rate suddenly exceeds aspecified threshold heart rate. The term specified refers to a parameterbeing a hard-set fixed value as well as being a programmable parameterwhose value is set with a device programmer. If a patient experiencestachyarrhythmia at a heart rate below the specified threshold heart rateor below typical cutoff rates for detection (i.e., a slowtachyarrhythmia), the tachyarrhythmia is not detected and treatment istherefore not provided. Some patients that have an implantable cardiacfunction management (CFM) device may experience episodes of slowtachyarrhythmia more frequently over time after an ICD is implanted. Thepresent inventors have recognized a need to detect slow tachyarrhythmia.The present inventors have also recognized a need to provide a warningsystem to identify when a patient is experiencing slow tachyarrhythmiaand to notify a caregiver.

Typically, ICDs divide the spectrum of possible heart rates into zones.For example, if an ICD detects that a heart rate falls within a zonethat defines ventricular tachycardia, the ICD may then trigger otherdetection methods to confirm that a patient is indeed experiencingventricular tachycardia. Detecting tachyarrhythmia without relying onthese heart rate zones allows an IMD to detect tachyarrhythmia at slowerheart rates than a specified heart rate.

Some CFMs include sensors that determine whether to increase or decreasea pacing rate. These devices often have a maximum sensor rate (MSR) thatis the maximum rate at which the device is allowed to pace the heart inresponse to the output of the sensor. Also, these devices often have amaximum tracking rate (MTR) that is the maximum rate at which the deviceis allowed pace the ventricle to maintain tracking with the atrium. Thegreater of the MSR and MTR represents the maximum pacing rate of thedevice. This rate is typically set below the tachyarrhythmia heart ratezones in order to avoid pacing the patient into such zones. Detectingtachyarrhythmia without using heart rate zones allows an IMD to detecttachyarrhythmia at rates below the maximum pace rate.

FIG. 1 illustrates an example of portions of a system 100 that uses animplantable medical device (IMD) 110. The system 100 shown is used totreat a cardiac arrhythmia. The IMD 110 includes an electronics unitcoupled by a cardiac lead 108, or additional leads, to a heart 105 of apatient 102. Examples of IMD 110 include, without limitation, a pacer, adefibrillator, a cardiac resynchronization therapy (CRT) device, or acombination of such devices. System 100 also typically includes an IMDprogrammer or other external device 170 that communicates wirelesssignals 160 with the IMD 110, such as by using radio frequency (RF) orother telemetry signals.

Cardiac lead 108 includes a proximal end that is coupled to IMD 110 anda distal end, coupled by electrical contacts called “electrodes” to oneor more portions of a heart 105. The electrodes typically delivercardioversion, defibrillation, pacing, or resynchronization therapy, orcombinations thereof to at least one chamber of the heart 105. Theelectronics unit of the IMD 110 typically includes components that areenclosed in a hermetically-sealed canister or “can.” Other electrodesmay be located on the can, or on an insulating header extending from thecan, or on other portions of IMD 110, such as for providing pacingenergy, defibrillation energy, or both, in conjunction with theelectrodes disposed on or around a heart 105. The lead 108 or leads andelectrodes may also typically be used for sensing electrical activity ofthe heart 105, including electrical activity related to contractions ofthe atria or ventricles.

FIGS. 2A-B illustrate IMDs 110 coupled by one or more leads 108A-C toheart 105. Heart 105 includes a right atrium 200A, a left atrium 200B, aright ventricle 205A, a left ventricle 205B, and a coronary sinus 220extending from right atrium 200A. In the example in FIG. 2A, atrial lead108A includes electrodes (electrical contacts, such as ring electrode225 and tip electrode 230) disposed in an atrium 200A of heart 105 forsensing signals, or delivering pacing therapy, or both, to the atrium200A.

Ventricular lead 108B includes one or more electrodes, such as tipelectrode 235 and ring electrode 240, for sensing signals, deliveringpacing therapy, or both sensing signals and delivering pacing therapy.Lead 108B optionally also includes additional electrodes, such as fordelivering atrial cardioversion, atrial defibrillation, ventricularcardioversion, ventricular defibrillation, or combinations thereof toheart 105. Such electrodes typically have larger surface areas thanpacing electrodes in order to handle the larger energies involved indefibrillation. Lead 108B optionally provides resynchronization therapyto the heart 105. The example in FIG. 2B includes a third cardiac lead108C attached to the IMD 110 through the header 255. The third lead 108Cincludes ring electrodes 260 and 265 placed in a coronary vein lyingepicardially on the left ventricle (LV) 205B via the coronary vein 220.

In the example of FIG. 2B, lead 108B further includes a firstdefibrillation coil electrode 275 located proximal to tip and ringelectrodes 235, 240 for placement in a right ventricle (RV), and asecond defibrillation coil electrode 280 for placement in the superiorvena cava (SVC) located proximal to the first defibrillation coil 275,tip electrode 235, and ring electrode 240. In some examples, high energyshock therapy is delivered from the first or RV coil 275 to the secondor SVC coil 280. In some examples, the SVC coil 280 is electrically tiedto an electrode formed on the IMD can 250. This improves defibrillationby delivering current from the RV coil 275 more uniformly over theventricular myocardium. In some examples, the therapy is delivered fromthe RV coil 275 only to the electrode formed on the IMD can 250.

Other forms of electrodes include meshes and patches which may beapplied to portions of heart 105 or which may be implanted in otherareas of the body to help “steer” electrical currents produced by IMD110. The present methods and systems will work in a variety ofconfigurations and with a variety of electrodes.

FIGS. 3A-B show an example of an IMD 300 that does not use intravascularleads to sense cardiac signals. FIG. 3A shows that the IMD 300 includesa thicker end 313 to hold the power source and circuits. The IMD 300also includes electrodes 325 and 327 for remote sensing of cardiacsignals. Cardioversion/defibrillation is provided through electrodes 315and 317. FIG. 3B shows the positioning of the IMD 300 within a patient.

FIG. 4 shows an example of a system 400 that detects tachyarrhythmia,including detecting tachyarrhythmia at rates below typicaltachyarrhythmia detection cutoff rates. The system 400 includes an IMD405, which in turn includes a ventricular contraction sensing circuit410, a timer circuit 415, and a controller circuit 420. The ventricularcontraction sensing circuit 410 provides a sensed ventricularcontraction signal. In the example shown, the signal is sensed betweenlead tip electrode 425 and lead ring electrode 430. In some examples thesignal is sensed between one lead electrode and a shock electrode placedin the right ventricle (RV). The timer circuit 415 is operable toprovide a ventricular time interval between contractions of a ventricleor ventricles. The controller circuit 420 is coupled to the timercircuit 415 and is operable to determine the ventricular contractionrate using the sensed ventricular contraction signal and the ventriculartime interval, such as by executing an algorithm or algorithmsimplemented by hardware, software, firmware or any combination ofhardware, software or firmware.

The controller circuit 420 further includes a tachyarrhythmia detectionmodule 435 that declares tachyarrhythmia when detecting a sudden rateincrease. A sudden rate increase is typically represented by a specifiednumber of consecutive accelerated beats. The number can be specified bybeing a hard fixed number or by being a programmable number programmedwith a device programmer. As an illustrative example, a sudden rateincrease is defined as three consecutive accelerated beats. In anotherexample, a sudden rate increase is defined as six consecutiveaccelerated beats. An accelerated beat is declared by some criterionother than a comparison to a fixed tachyarrhythmia rate threshold (i.e.,without a comparison to one or more tachyarrhythmia heart rate zones orone or more tachyarrhythmia detection cutoff heart rates). In someexamples, an accelerated beat is identified when a difference between alast average ventricular contraction interval (V-V interval) and thecurrent V-V interval is greater than a specified percentage of the lastaverage V-V interval. As an illustrative example, an accelerated beat isidentified when the difference between the last average V-V interval andthe current V-V interval is greater than ten percent (10%) of the lastaverage V-V interval, i.e.,VV _(AVG)(n−1)−VV(n)>(0.1)*(VV _(AVG)(n−1)),where VV_(AVG)(n−1) is the last or previous average V-V interval andVV(n) is the current V-V interval. If a current beat is not anaccelerated beat, the current V-V interval will be close to the averageinterval and the difference will be close to zero. If the current beatis an accelerated beat, the current V-V interval will be smaller thanthe average interval value and the difference will increase to aquantity larger than zero. As the current V-V interval decreases,eventually the difference will exceed the specified percentagedifference and the tachyarrhythmia detection module 435 identifies thebeat as an accelerated beat.

There is a complication in calculating accelerated beats. If theaccelerated beat intervals are included in the calculation of theaverage V-V interval, the accelerated beats will skew the average tofaster intervals or a fast interval steady state if the sudden rateincrease is sustained. This is illustrated in FIG. 5. An onset ofaccelerated beats that are sustained is shown by graph 505. Graph 510shows the effect on the average V-V interval. Graph 515 shows0.9*(average V-V interval). It can be seen in the graphs that the fastV-V intervals and the average V-V interval converge, making it difficultfor subsequent accelerated beats to be detected. In some examples, thiscomplication is overcome by the controller circuit 420 not including theaccelerated beats in the calculation of the average V-V interval. Inanother example, any accelerated beat that occurs after a specifiednumber of accelerated beats, such as three accelerated beats forexample, is not used to update the average V-V interval.

In another example, the controller circuit 420 calculates a temporaryaverage V-V interval, different from the last average V-V interval,using the number of consecutive accelerated beats until a sudden rateincrease is defined. The temporary average V-V interval is not updatedlike the normal average V-V interval and is used to identify anaccelerated beat after the average V-V interval converges to fast V-Vintervals. For example, when an initial sudden rate increase isdeclared, such as after detecting a third consecutive accelerated beat,the average V-V interval is updated but becomes a temporary average V-Vinterval. The normal average V-V interval is preserved. Any V-V intervalshorter than the temporary V-V interval is deemed to be an acceleratedbeat.

In some examples, the IMD 405 includes an atrial contraction sensingcircuit configured to sense a cardiac signal of an atrium of the heart.The controller circuit 420 monitors the ventricular and atrialcontraction rate and the tachyarrhythmia detection module 435 declarestachyarrhythmia when the ventricular contraction rate exceeds the atrialcontraction rate by a specified rate difference threshold. As anillustrative example, the tachyarrhythmia detection module 435 declarestachyarrhythmia when the ventricular contraction rate exceeds the atrialcontraction rate by ten beats per minute (bpm). In some examples, thetachyarrhythmia detection module 435 declares tachyarrhythmia wheneither the ventricular contraction rate exceeds the atrial contractionrate or when a sudden rate increase is detected. In some examples, thetachyarrhythmia detection module 435 declares tachyarrhythmia when boththe ventricular contraction rate exceeds the atrial contraction rate anda sudden rate increase is detected.

In some examples, the controller circuit 420 includes a rhythmdiscrimination module 440 that discriminates between ventriculartachyarrhythmia (such as ventricular tachycardia (VT), or ventricularfibrillation (VF), or both VT and VF, for example) and supraventriculartachyarrhythmia. In some examples, the controller circuit 420 enablesthe rhythm discrimination module 440 when the controller circuit 420detects that the detected sudden rate increase is sustained for aspecified period of time. The specified period of time may be measuredin seconds (e.g., ten seconds) or it may be measured in heartbeats. Asan illustrative example, the controller circuit 420 detects that asudden rate increase is sustained if eight out of ten heartbeats areaccelerated beats. In some examples, the controller circuit 420 enablesthe rhythm discrimination module 440 when the controller circuit 420detects that the ventricular rate exceeds the atrial rate by a specifiedrate difference threshold and that this ventricular rate is sustainedfor a specified period of time. Again, the controller circuit 420 maymeasure the specified period of time in seconds or in heartbeats. Insome examples, the controller circuit 420 enables the rhythmdiscrimination module 440 when either the controller circuit 420 detectsthat both the sudden rate increase or the ventricular rate that exceedsthe atrial rate are sustained for a specified period of time.

The rhythm discrimination module 440 typically discriminates betweenventricular tachyarrhythmia and supraventricular tachyarrhythmia byperforming different techniques than are performed by the detectionmodule 435. In this way the tachyarrhythmia detection module 435 can beconfigured to increase the sensitivity of a detection device and therhythm discrimination module 440 can be configured to increase thespecificity of a detection device.

Sensitivity generally refers to the ability of the detection scheme toeffectively detect an abnormal heart rhythm (e.g., VT/VF) that thephysician desires the cardiac rhythm management device to treat. Thesensitivity can be expressed as follows:Sensitivity=True Positives/(True Positives+False Negatives).Specificity generally refers to the ability of the detection scheme toavoid improperly treating rhythms (e.g., sinus tachycardia) that thephysician determines that the device should not treat. The specificitycan be expressed as follows:Specificity=True Negatives/(True Negatives+False Positives).For example, if the rhythm to be detected is VT/VF, then a true positivewould occur when a particular rhythm is VT/VF and the detectionalgorithm correctly declares it as VT/VF. A false negative would occurwhen the rhythm is VT/VF and the detection algorithm erroneouslydeclares it as not VT/VF. A false positive would occur when the rhythmis anything but VT/VF (e.g., normal sinus rhythm (NSR), sinustachycardia, atrial fibrillation, atrial flutter, electrical noise,e.g., due to mypotentials, electromagnetic interference (EMI), a looseset screw for a leadwire, a broken leadwire, etc.) and the detectionalgorithm erroneously declares it as VT/VF. A true negative would occurwhen the rhythm is anything but VT/VF (e.g., normal sinus rhythm (NSR),sinus tachycardia, atrial fibrillation, atrial flutter, electricalnoise, e.g., due to mypotentials, electromagnetic interference (EMI), aloose set screw for a leadwire, a broken leadwire, etc.) and thedetection algorithm correctly declares it as not VT/VF.

In some examples, the rhythm discrimination module 440 performs a rhythmdiscrimination method that includes recurrently updating an averageventricular contraction interval (V-V interval) and determining that anaverage ventricular contraction rate exceeds an average atrialcontraction rate by more than a specified rate threshold value. Thisdiffers from the atrial/ventricular rate detection performed by thetachyarrhythmia detection module 435 in that averages of the atrial andventricular rate are used to confirm the tachyarrhythmia anddiscriminate from supraventricular tachyarrhythmia. Descriptions ofsystems and methods for classifying detected tachycardia based onaverage atrial and ventricular rates calculated from selected atrial andventricular intervals is found in co-pending U.S. patent applicationSer. No. 11/054,726, Elahi et al., entitled, “Method and Apparatus forRate Accuracy Enhancement in Ventricular Tachycardia Detection,” filedFeb. 10, 2005, which is incorporated herein by reference.

In some examples, the controller circuit includes a memory and therhythm discrimination module 440 performs a morphology comparison of asensed cardiac signal to a template of a known morphology (such asventricular tachyarrhythmia or supraventricular tachyarrhythmia) storedin memory. For example, a template can be created for a patient using aCRM by providing electrical energy pulses to the supraventricular regionof the patient's heart. The resulting cardiac complexes are then sensedand used to create a template for use in a morphology-based cardiacclassification algorithm for classifying cardiac complexes as either VTor SVT. Systems and methods of creating templates for a morphology-basedalgorithm are described in Hsu, U.S. Pat. No. 6,889,081, entitled“Classification of Supraventricular and Ventricular Cardiac RhythmsUsing Cross Channel Timing Algorithm,” filed Jul. 23, 2002, which isincorporated herein by reference.

In another example, a template is generated from a snapshotrepresentative of one of the patient's normal supra-ventricularconducted beats. Cardiac signals are sensed from pacing leads (ratechannel) and shock leads (shock channel). A fiducial point is determinedfrom the signals sensed on the rate channels and is used to alignsignals sensed on the shock channels. A template for a patient isgenerated using the aligned shock channel signals. The template isrepresentative of one of the patient's normal supra-ventricularconducted beats. Subsequently detected beats are then used to confirmthat the generated template is representative of one of the patient'snormal supra-ventricular conducted beats. Systems and methods forgenerating templates using a snapshot of the patient's normalsupra-ventricular conducted beats are described in Kim et al., U.S. Pat.No. 6,708,058, entitled “Normal Cardiac Rhythm Template GenerationSystem and Method,” filed Apr. 30, 2001, which is incorporated herein byreference.

In another example, a template of a patient's supraventricular rhythm isgenerated from characterizations performed while the heart is beingpaced. During the characterization, various pacing parameters aremodified and the patient's supraventricular rhythm is characterizedwhile the pacing parameters are modified. Systems and methods forgenerating a template to represent a patient's supraventricular rhythmare described in Bocek et al., U.S. Pat. No. 6,889,079, entitled “Methodand System for Characterizing Supraventricular Rhythm During CardiacPacing,” filed Apr. 12, 2002, which is incorporated herein by reference.

In some examples, the rhythm discrimination module 440 performs a rhythmdiscrimination method that includes determining that a specified numberof atrial rate intervals measured over a consecutive number of heartbeats are shorter than an atrial fibrillation rate threshold interval.In an example, at initiation of ventricular tachyarrhythmia, each atrialinterval is classified as faster or slower than the atrial fibrillationrate threshold interval. When 6 of the last 10 intervals are classifiedas faster than the atrial fibrillation rate threshold interval, thedevice declares atrial fibrillation present.

In some examples, the rhythm discrimination module 440 performs a rhythmdiscrimination method that includes assessing stability of theventricular rhythm. In an example, the stability is assessed bymeasuring the degree of variability of R-R intervals during thetachycardia episode. The current average difference between R-Rintervals is compared to a programmed stability threshold and a “shockif unstable” threshold. If the average difference is greater than theprogrammed thresholds, the rhythm is declared unstable. Descriptions ofadvanced methods and systems to detect abnormal heart rhythms includingdetermining that a specified number of atrial rate intervals measuredover a consecutive number of heart beats are shorter than an atrialfibrillation rate threshold interval and assessing the stability of theventricular rhythm are found in Gilkerson et al., U.S. Pat. No.6,493,579, entitled “System and Method for Detection EnhancementProgramming,” filed Aug. 20, 1999, which is incorporated herein byreference.

The stability can be assessed by determining whether the ventricularrhythm is unstable using a measure of variability of ventricular timeintervals, or the stability can be assessed from the variability of therate in combination with measurements of other physiologic measurements,such as a hemodynamic pressure sensor for example.

In some examples, the rhythm discrimination module 440 uses acombination of morphology discrimination and rhythm discrimination toclassify rhythms. In an example, the rhythm discrimination module mayperform a morphology discrimination if a conflict among initial rhythmdiscriminators exists. Additional discrimination procedures can be usedto enhance rhythm discrimination from additional information availablein the system 400. For example, CRM devices often include a number ofsensors that are used for diagnostic or therapeutic purposes. Sensorinformation acquired from such sensor components may be used with rhythmclassification. Systems and methods for classifying cardiac rhythms byblending rhythm discriminators are described in Kim et al., U.S. patentapplication Ser. No. 11/089,185, entitled, “Blending Cardiac RhythmDetection Processes,” filed Mar. 24, 2005, which is incorporated hereinby reference.

In some examples, multiple morphology templates are used in conjunctionwith rate discrimination. In an example, a morphology template for restand a morphology template for exercise are used in conjunction withheart rate to classify rhythms. In a further example, a patient'smetabolic need provides further information for such a classification.Systems and methods that use multiple morphology templates todiscriminate between rhythms are found in Schwartz et al, U.S. Pat Appl.Pub. No. 20040093035, filed Nov. 8, 2002, which is incorporated hereinby reference.

Once it determined that the tachyarrhythmia is ventriculartachyarrhythmia and not supraventricular tachyarrhythmia, some CFMdevices treat the ventricular tachyarrhythmia. In some examples, the IMD405 includes a therapy circuit coupled to the controller circuit 420.The therapy circuit delivers anti-tachyarrhythmia therapy when therhythm discrimination module detects ventricular tachyarrhythmia.

In some examples, the IMD 405 is configurable to deliveranti-tachyarrhythmia therapy in the ventricle when the rhythmdiscrimination module 440 detects a slow ventricular tachyarrhythmia.This ventricular anti-tachyarrhythmia therapy can be selectivelyprogrammed by a physician to deliver either ventricular anti-tachycardiapacing (ATP) or ventricular defibrillation/cardioversion shocks, or acombination of both. In some examples, the rhythm discrimination module440 can be configured to discriminate between normal sinus tachycardiaand atrial tachyarrhythmias, such as by comparing the sensed atrial rateto an atrial tachyarrhythmia rate threshold that the physician programsto be above the patient's maximum sinus rate. In some examples, theIMD's 405 atrial anti-tachyarrhythmia therapy can be selectivelyprogrammed by the physician to deliver either atrial ATP or atrialdefibrillation/cardioversion shocks or a combination of both in responseto detection of an atrial tachyarrhythmia.

Anti-tachyarrhythmia therapy can result in perceived discomfort oracceleration of the tachyarrhythmia to higher rates that are poorlytolerated by the patient. For this reason, optimal therapy may be towithhold device therapy for slow tachyarrhythmias for which the patienthas acceptable hemodynamic response. In some examples, atrial and/orventricular therapy is programmable to be enabled only after evidence ofhemodynamic compromise, such as by blood pressure falling below aphysician programmable threshold as measured by an implantable pressuresensor for example. If a patient tolerates the presence of slowtachyarrhythmia for a period of time, device therapy may be delayed toallow the rhythm to self-terminate or to allow pharmacologicalinterventions to be applied. Atrial and/or ventricular therapy may beprogrammed to activate following a physician programmable sustainedduration of the slow VT (e.g. in a range of minutes to several hours).

ATP is implemented by providing one or more bursts of pacing pulses inone or more atria for atrial therapy, and one or more ventricles forventricular therapy. The therapy may include programmable sequences ofbursts with programmable amplitude, width, length, pacing interval, andcoupling interval (time from a sensed beat of the tachycardia to theinitial pulse in a burst). A ramp feature may be programmed thatprogressively shortens the pacing interval during each burst. A scanfeature may be programmed to progressively that shortens the pacinginterval from one burst to the next. Ramp and scan features may beprogrammed in combination. Pacing interval and coupling interval may beprogrammed to be adaptive to the sensed rate of the tachyarrhythmia. Thephysician may program multiple anti-tachycardia pacing sequences to beapplied consecutively to a single tachyarrhythmia. If multiple burstsare programmed, redetection is applied after each burst to determine ifthe therapy has converted the tachyarrhythmia, or has resulted inacceleration of the arrhythmia to a higher rate, e.g. into a rate zonefor which other therapy has been programmed. A programmable timeout maybe provided to limit the total duration of ATP for a given episode oftachyarrhythmia. After all programmed ATP is delivered or the timeout isreached, ATP is exhausted for the episode. If both shock and ATPtherapies are programmed, ATP is delivered first to minimize patientdiscomfort.

Ventricular or atrial shock therapy consists of delivery of one or moreshocks of programmed energy levels. If multiple shocks are programmed,successive shocks are normally constrained to have the same or increasedenergy level. If multiple shocks are programmed, redetection is appliedafter each burst to determine if the therapy has converted thetachyarrhythmia, or has resulted in acceleration of the arrhythmia to ahigher rate, e.g. into a rate zone for which other therapy has beenprogrammed. Shocks are normally delivered synchronously with a senseddepolarization of the ventricle. Atrial shocks are frequentlyconstrained to be delivered only on intervals longer than apredetermined or computed minimum ventricular interval, to avoidventricular pro-arrhythmia due to shock delivery on the ventricularvulnerable period. Before each shock, the defibrillator typicallyrequires several seconds to charge internal energy storage capacitors tothe programmed energy for that shock. Ventricular shocks may beprogrammed to be committed or uncommitted. For committed shocks, shockdelivery takes place at the end of charging without regard to thepatient's sense rate at the time charging completes place. Foruncommitted shocks, the shock is withheld if the patient's rate is notfound to be high at the end of charging, and redetection follows. Afterall programmed shocks have been delivered, shock therapy is exhaustedfor the episode.

FIG. 6 shows another example of a system 600 that detectstachyarrhythmia, including detecting tachyarrhythmia at rates belowtypical detection cutoff rates. The system 600 includes an IMD 605 andan external device 670. The IMD 605 in turn includes a ventricularcontraction sensing circuit 610, an atrial contraction sensing circuit645, a timer circuit 615, and a controller circuit 620. The controllercircuit 620 includes a tachyarrhythmia detection module 635 and a rhythmdiscrimination module 640. The ventricular contraction sensing circuit610 provides a ventricular contraction signal sensed through lead tipelectrode 625 and lead ring electrode 630. The atrial contractionsensing circuit 645 provides an atrial contraction signal sensed throughlead tip electrode 650 and lead ring electrode 655.

The system 600 includes a memory circuit 660 coupled to the controllercircuit 620. In some examples, when the rhythm discrimination module 640identifies an episode of tachyarrhythmia such as ventriculartachyarrhythmia or supraventricular tachyarrhythmia, the controllercircuit 620 stores a digitized representation of the episode in thememory circuit 660. The digitized representation of the episode isobtained using sampling circuits to sample the electrical cardiacsignals sensed by the sensing circuits 610, 645. In some examples, thecontroller circuit 620 stores episodes of ventricular tachyarrhythmia ina first area of memory and episodes of supraventricular tachyarrhythmiain a second area of memory, where “area of memory” refers to memorylocations defined by a range of addresses.

It should be noted that the previously mentioned examples are capable ofdetecting tachyarrhythmia without first comparing a heart rate to afixed tachyarrhythmia detection threshold rate or set of threshold rates(i.e., rate zones). These fixed rates are sometimes programmable. Someexamples of these tachyarrhythmia rates include 140 beats per minute(bpm), 160 bpm, and 180 bpm. Because the examples do not require acomparison to a fixed tachyarrhythmia rate, tachyarrhythmia occurringbelow these rates can be detected. In addition, this allows detection oftachyarrhythmia at rates below a maximum pace rate.

However, after tachyarrhythmia detection, a comparison to these ratescan provide information to a clinician that a patient is experiencingslow tachyarrhythmia. In some examples, the controller circuit 620 deemsan episode of tachyarrhythmia as an episode of slow tachyarrhythmia whenthe tachyarrhythmia occurs at a ventricular rate less than a specifiedtachyarrhythmia detection rate. At least one digitized representation ofan episode of slow tachyarrhythmia can be stored by the controllercircuit 620 in the memory circuit 660. In some examples, a plurality ofslow tachyarrhythmia episode representations are stored in the memorycircuit 660 and the controller circuit 620 overwrites the oldest storedslow tachyarrhythmia episode representation with the most recent episoderepresentation when memory storage available for slow tachyarrhythmiaepisode representations is full.

In some examples, the IMD 605 further includes a communication circuit665 and the controller circuit 620 wirelessly communicates informationrelated to a detected slow tachyarrhythmia to the external device 670.In some examples, the external device 670 is part of, or incommunication with, a computer network such as a hospital computernetwork or the internet. In some examples, the external device 670 is incommunication with a mobile telephone network. In some examples, theexternal device is a repeater that communicates wirelessly with the IMDand with a third device in communication with a network, such as acomputer network or mobile telephone network.

The controller circuit 620 communicates an indication of a detected slowtachyarrhythmia to the external device 670. The indication can be theentire digitized representation for display on the external device 670or another device connected to the network, or the indication can be analarm on the external device 670. In some examples, the alarm includes anotification sent to a clinician or clinician's office over the computernetwork, such as by e-mail, or the alarm includes an indication on a webpage. In some examples, the alarm includes an indication or notificationsent to a medical device service center. In some examples, the IMD 605includes a speaker and the indication of slow tachyarrhythmia is anaudible alarm originating from the IMD 605.

FIG. 7 is a block diagram of an example of a method 700 of using an IMD.At 710, ventricular contraction intervals (V-V intervals) of a subjectare monitored. The intervals are monitored using cardiac signal sensingcircuits. At 720, a sudden rate increase in the V-V intervals isdetected. A sudden rate increase is declared by a specified number ofconsecutive accelerated beats. An accelerated beat typically is a V-Vinterval that is shorter than one or more previous V-V intervals by athreshold value.

In some examples of the method 700, detecting a sudden rate increase inthe V-V intervals includes recurrently updating an average V-V intervaland deeming that a current V-V interval indicates an accelerated beat ifa difference between a last average V-V interval and the current V-Vinterval exceeds a specified percentage of the last average V-Vinterval. In some examples, recurrently updating an average V-V intervalincludes recurrently calculating an average V-V interval and apreviously calculated average interval is given a greater weight in thecalculation than a current rate interval. As an illustrative example, anaverage V-V interval, V-V_(AVG)(n) is calculated byVV _(AVG)(n)=0.75*VV _(AVG)(n−1)+0.25*VV(n),where VV(n) is the current rate interval. In some examples, the currentV-V interval is excluded from an updated average V-V intervalcalculation if the current V-V interval indicates an accelerated beat.This prevents the accelerated beats from skewing the interval averagetoward the faster intervals if the sudden rate increase is sustained. Inanother example, the V-V interval that indicates an accelerated beat isnot excluded from the updated average V-V interval until a sudden rateincrease is declared, i.e., until a specified number of acceleratedbeats is detected. In an illustrative example, a current fast V-Vinterval is not excluded from the updated average until threeconsecutive accelerated beats are first detected.

In another example, once the specified number of accelerated beatsneeded to declare a sudden rate increase is detected, subsequentaccelerated beats are used to define a temporary average V-V interval.The temporary average V-V interval is not updated in the same manner asthe normal average V-V interval, but is used to identify acceleratedbeats after the average V-V interval converges to fast V-V intervals.For example, when a sudden rate increase is declared after threeaccelerated beats are detected, the current V-V interval and the normalaverage V-V interval are used to calculate a temporary average V-Vinterval. Subsequent V-V intervals shorter than the temporary averageV-V interval are deemed to be accelerated beats.

At 730, whether the sudden rate increase is sustained for a specifiedperiod of time is determined. In some examples, determining that asudden rate increase is sustained for a specified period of timeincludes determining that the accelerated beats are sustained over A ofB ventricular time intervals, where A and B are integers, and A≦B. Forexample, a sudden rate increase is determined to be sustained if 8 outof 10 ventricular time intervals are accelerated beats (i.e., A=8 andB=10). At 740, the sudden rate increase is deemed to indicatetachyarrhythmia if the sudden rate increase is sustained. The end of thetachyarrhythmia episode is detected if the fast time intervals aresustained over less than C of D V-V intervals, where C and D areintegers, and C<D. For example, if an episode tachyarrhythmia isdeclared when 8 out of 10 ventricular time intervals are acceleratedbeats, the episode is declared to be over if 4 out of 10 ventriculartime intervals are accelerated beats (i.e., C=4 and D=10).

According to some examples, the method 700 further includes, at 750,monitoring an atrial contraction rate (A-A intervals) of the subject. At760, whether the ventricular contraction rate exceeds the subject'satrial contraction rate by more than specified rate threshold isdetected. In some examples, the specified rate threshold is ten bpm. At770, tachyarrhythmia is declared if the sudden rate increase issustained for a specified period of time, or if the ventricularcontraction rate, which exceeds the atrial contraction rate by more thanspecified rate threshold, is sustained for a specified period of time.Thus, in some examples tachyarrhythmia is detected only from adetermination that the ventricular rate exceeds the atrial rate, andwithout detecting a sudden rate increase. Determining whether the highventricular contraction rate is sustained can be based on time (e.g.,tachyarrhythmia is declared if the rate is sustained for ten seconds) orit can be based on a number of heart beats (e.g., tachyarrhythmia isdeclared if the ventricular rate interval is less than the atrial rateinterval over ten ventricular contractions). In some examples,tachyarrhythmia is declared only if both the sudden rate increase andthe ventricular rate are sustained for a specified period of time.

In some examples of the method 700, once tachyarrhythmia is detected, itis determined whether the tachyarrhythmia is ventricular tachyarrhythmiaheart rhythms or supraventricular tachyarrhythmia heart rhythms. This isaccomplished by using one or more advanced algorithms to discriminatethe type of heart rhythm. These algorithms include determining that anaverage ventricular contraction rate exceeds an average atrialcontraction rate by more than a specified rate threshold value,comparing a morphology of a sensed cardiac signal to a templatemorphology, determining that an atrial rate exceeds an atrialfibrillation rate threshold, and assessing the stability of theventricular rhythm. In some examples, the stability is assessed bydetermining that the ventricular rhythm is unstable using a measure ofvariability of the ventricular time intervals. In some examples, thestability is assessed from variability of the intervals in combination ameasurement of another physiologic parameter.

Because tachyarrhythmia is detected without first comparing a heart rateto a fixed tachyarrhythmia detection threshold rate or set of thresholdrates, after tachyarrhythmia is detected a subsequent comparison to sucha rate can determine if the patient experienced tachyarrhythmia at arelatively low heart rate for tachyarrhythmia, such as 140 bpm forexample. Therefore, in some examples of the method 700, an episode oftachyarrhythmia is deemed to be slow tachyarrhythmia when theventricular rate is less than a specified tachyarrhythmia detectionrate. In some examples, the method includes communicating an indicationof slow tachyarrhythmia from the IMD to an external device.

In some examples, at least one digitized representation of an episode ofslow tachyarrhythmia is stored in memory of the IMD. The digitizedrepresentations of the episode are obtained by sampling of electricalcardiac signals sensed using the IMD. These digitized representationscan then be displayed. This allows a caregiver to read a sampled signalout from device memory and observe the episode of slow tachyarrhythmiaat a subsequent patient visit. An indication in the IMD is communicatedto an external device to alert the caregiver of the episode. In someexamples, an external device located somewhere other than in a clinic,such as in a patient's home, can read out and communicate the sampledsignal to the caregiver's location for display. This monitoringidentifies those patients that experience slow tachyarrhythmia to acaregiver, allowing the caregiver to make adjustments to parameters of aCFM device or to adjust a patient's drug therapy.

In versions of the example, a plurality of episodes of slowtachyarrhythmia are stored in the IMD and, as the storage available forslow tachyarrhythmia episodes is filled, the oldest stored episode isoverwritten with a most recent episode. In some examples, episodes ofsupraventricular tachyarrhythmia are also obtained and stored withepisodes of slow tachyarrhythmia stored in a separate area of memoryfrom the supraventricular episodes.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific examples in which thesubject matter may be practiced. The examples illustrated are describedin sufficient detail to enable those skilled in the art to practice theteachings disclosed herein. Other examples may be utilized and derivedtherefrom, such that structural and logical substitutions and changesmay be made without departing from the scope of this disclosure. ThisDetailed Description, therefore, is not to be taken in a limiting sense,and the scope of various examples is defined only by the appendedclaims, along with the full range of equivalents to which such claimsare entitled.

Such examples of the inventive subject matter may be referred to herein,individually and/or collectively, by the term “invention” merely forconvenience and without intending to voluntarily limit the scope of thisapplication to any single invention or inventive concept if more thanone is in fact disclosed. Thus, although specific examples have beenillustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific examples shown. This disclosure is intended to coverany and all adaptations, or variations, or combinations of variousexamples. Combinations of the above examples, and other examples notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single example for the purpose of streamlining the disclosure. Thismethod of disclosure is not to be interpreted as reflecting an intentionthat the claimed examples require more features than are expresslyrecited in each claim. Rather, as the following claims reflect,inventive subject matter lies in less than all features of a singledisclosed example. Thus the following claims are hereby incorporatedinto the Detailed Description, with each claim standing on its own.

1. A method of using an implantable medical device (IMD) comprising:monitoring a ventricular contraction rate of a subject; monitoring anatrial contraction rate of the subject; declaring tachyarrhythmia if theventricular contraction rate exceeds the atrial contraction rate; anddeclaring a slow tachyarrhythmia when the ventricular contraction rateexceeds the atrial contraction rate and the ventricular rate is lessthan a specified lowest tachyarrhythmia detection rate.
 2. The method ofclaim 1, wherein detecting that the ventricular contraction rate exceedsthe atrial contraction rate includes: calculating an average ventricularrate and an average atrial rate; and deeming that the ventricularcontraction rate exceeds the atrial contraction rate when the averageventricular rate is faster than the average atrial rate by more than aspecified rate threshold value.
 3. The method of claim 2, whereincalculating an average ventricular rate includes recurrently updating anaverage ventricular rate interval by summing a weighted previous averageventricular rate interval with a weighted current ventricular rateinterval.
 4. The method of claim 2, wherein calculating an averageatrial rate includes recurrently updating an average atrial rateinterval by summing a weighted previous average atrial rate intervalwith a weighted current atrial rate interval.
 5. The method of claim 1,including discriminating between ventricular tachyarrhythmia andsupraventricular tachyarrhythmia using at least one of: (a) comparing amorphology of a sensed cardiac signal to a template morphology; (b)determining that an atrial rate exceeds an atrial fibrillation ratethreshold; and (c) assessing the stability of the ventricular rhythm. 6.The method of claim 1, including discriminating between ventriculartachyarrhythmia and supraventricular tachyarrhythmia when theventricular contraction rate of the subject exceeds the subject's atrialcontraction rate, or the ventricular contraction interval is less thanthe atrial contraction interval, for a specified period of time.
 7. Themethod of claim 1, including communicating an indication of the slowtachyarrhythmia from the IMD.
 8. The method of claim 1, includingstoring at least one episode of slow tachyarrhythmia in the IMD.
 9. Themethod of claim 8, including: storing a plurality of episodes of slowtachyarrhythmia in the IMD; and overwriting an oldest stored episodewith a most recent episode when the storage available for episodes isfull.
 10. The method of claim 1, wherein declaring tachyarrhythmiaincludes declaring tachyarrhythmia when the ventricular contraction rateexceeds the atrial contraction rate and a sudden rate increase isdetected, wherein a sudden rate increase is defined as a specifiednumber of consecutive accelerated beats, and wherein a current heartbeat is identified as an accelerated beat when a difference between alast average ventricular contraction interval (V-V interval) and thecurrent V-V interval is greater than a specified portion of the lastaverage V-V interval.
 11. An apparatus comprising: a ventricularcontraction sensing circuit configured to provide a sensed ventricularcontraction signal of a heart of a subject; an atrial contractionsensing circuit configured to provide a sensed atrial contractionsignal; a timer circuit operable to provide a ventricular time intervalbetween ventricular contractions and an atrial time interval betweenatrial contractions; and a controller circuit coupled to the timercircuit, the controller circuit configured to: monitor a ventricularcontraction rate or the ventricular time interval; and monitor an atrialcontraction rate or the atrial time interval, and wherein the controllercircuit further includes a tachyarrhythmia detection module configuredto: declare tachyarrhythmia when the ventricular contraction rateexceeds the atrial contraction rate or the ventricular time interval isless than the atrial time interval; and declare a slow tachyarrhythmiawhen the ventricular contraction rate exceeds the atrial contractionrate and either of the following: when the ventricular rate is less thana specified maximum pacing rate, or when the ventricular time intervalis more than a specified minimum specified pacing interval.
 12. Theapparatus of claim 11, wherein the tachyarrhythmia detection module isconfigured to: calculate an average ventricular rate or interval and anaverage atrial rate or interval; and declare the tachyarrhythmia whenthe average ventricular rate is faster than the average atrial rate bymore than a specified rate threshold value or the average ventricularinterval is less than the atrial interval by a specified intervalthreshold value.
 13. The apparatus of claim 12, wherein thetachyarrhythmia detection module is configured to calculate the averageventricular rate or interval by recurrently updating an averageventricular rate interval using a sum of a weighted previous averageventricular rate interval and a weighted current ventricular rateinterval.
 14. The apparatus of claim 12, wherein the tachyarrhythmiadetection module is configured to recurrently update an average atrialrate interval by summing a weighted previous average atrial rateinterval with a weighted current atrial rate interval.
 15. The apparatusof claim 11, wherein the tachyarrhythmia detection module is configuredto discriminate between ventricular tachyarrhythmia and supraventriculartachyarrhythmia using at least one of: (a) a comparison of a morphologyof a sensed cardiac signal to a template morphology; (b) a determinationthat an atrial rate or rate interval satisfies an atrial fibrillationrate or rate interval threshold; and (c) a stability of the ventricularrhythm.
 16. The apparatus of claim 11, wherein the tachyarrhythmiadetection module is configured to discriminate between ventriculartachyarrhythmia and supraventricular tachyarrhythmia when a ventricularcontraction rate of a subject exceeds the subject's atrial contractionrate for a specified period of time, or the ventricular contractioninterval is less than the atrial contraction interval for the specifiedperiod of time.
 17. The apparatus of claim 11, including a communicationcircuit coupled to the controller circuit, wherein the controllercircuit is configured to communicate an alert of a detected slowtachyarrhythmia to a second device.
 18. The apparatus of claim 11,including a memory circuit coupled to the controller circuit, the memorycircuit to store a digitized representation of at least one episode ofslow tachyarrhythmia.
 19. The apparatus of claim 18, wherein the memorycircuit is configured to store a plurality of episodes of slowtachyarrhythmia; and wherein the controller circuit is configured tooverwrite an oldest stored episode with a most recent episode when thestorage available for episodes is full.
 20. The apparatus of claim 11,wherein the tachyarrhythmia detection module is configured to: declaretachyarrhythmia when the ventricular contraction rate exceeds the atrialcontraction rate, or the ventricular time interval is less than theatrial time interval, and a sudden rate increase is detected; deem asudden rate increase as a specified number of consecutive acceleratedbeats; and identify a current heart beat as an accelerated beat when adifference between a last average ventricular contraction interval (V-Vinterval) and the current V-V interval is greater than a specifiedportion of the last average V-V interval.
 21. The apparatus of claim 11,wherein the tachyarrhythmia detection module is configured to declare aslow tachyarrhythmia when the ventricular contraction rate exceeds theatrial contraction rate and the ventricular rate is less than a lowestspecified tachyarrhythmia detection rate and a specified maximum pacingrate or the ventricular time interval is more than a specified longesttachyarrhythmia detection interval and a specified minimum pacinginterval.